1. Field of the Invention
The invention relates to a system for determining the flow performance characteristics of a flow control device and, more particularly, to determining the flow performance characteristics of a gas lift valve.
2. History of the Prior Art
Every device which is used as a flow restrictor or flow controller acccomplishes its purpose by putting some type of restriction into the flow path of the fluid being controlled. In such devices, there are basically three parameters which affect the characteristics of the flow through the device. These are the upstream pressure, the downstream pressure, and the degree of flow restriction through the device. The flow coefficient, C.sub.v, of a device is a measurement of its ability to restrict fluid flow through it. This value is independent of both absolute pressure of the device and the differential pressure across it. Rather, it is a function of only the geometry of the device itself. In the case where the device is an orifice valve where the geometry does not change, this factor is referred to as the discharge coefficient of the valve. In the case of a valve where the geometry does change depending on the position of the valve stem with respect to the seat, this value is called the flow coefficient of the valve. The flow coefficient of a valve changes as a function of the position of the valve stem with respect to the seat of the valve. When the valve is completely closed the flow coefficient is 0 and when the valve is full open its flow coefficient may be equal to 1. In between these two extremes, the flow coefficient of the valve may or may not vary linearly with the position of the valve stem and can have any particular shape. Ascertaining the shape of a valve's flow coefficient curve as a function of all the various parameters which can effect it in an operating flow environment, is difficult. Nevertheless, the flow characteristics of a valve are very important in predicting the performance of the valve in operation.
One particular environment in which the operating characteristics of a flow control device are highly critical to the successful operation of the device is in the case of gas lift valves used in a gas lift petroleum production well completion.
In a petroleum production well in which the native reservoir energy of the formations producing into the well are sufficiently low that there is not enough pressure within the formation to force fluids in the well to the surface, gases from an external source are often injected into the well casing and from there into the well tubing at various points spaced downwardly along the tubing to force the production fluids to the surface. In other wells in which there is sufficient reservoir pressure to force fluids to the surface, injection gases may often be used to increase the production from the well. This technique of producing fluids in a well is known as a gas lift well completion. The height of fluid within the well tubing which can be forced to the surface in a gas lift completion is a function of the amount of pressure available in the casing and the density of the fluids in the tubing. For this reason, gas lift injection valves are placed in the well tubing string at spaced intervals down the tubing to control the injection of gas from the casing into the tubing as a function of: (a) the pressure in the casing (b) the pressure in the tubing; and (c) the set pressure of the gas lift valve itself. Each valve generally includes a "set pressure" which is established by a chamber within the valve which is filled with compressed nitrogen to a preselected pressure value and which acts to exert a force tending to close the valve or by a compressed spring which controls the force of engagement of the stem of the valve with the seat also tending to close the valve. The pressure within the well tubing and the annulus applies a force opposite to that of the set pressure of the valve tending to open the valve. While there are various types of gas lift valves, e.g., production pressure operated valves and injection pressure operated valves, only the production pressure operated gas lift valves will be discussed hereinafter for purposes of illustration. When the pressure within the tubing reaches a preselected value, established in accordance with the set pressure of a production pressure operated gas lift valve, the valve will open to inject pressurized gas from within the well casing into the tubing so as to lift the fluids which have collected in the tubing above that valve toward the surface and discharge them out into a collection reservoir. Once the fluids above each gas lift valve have been aerated to decrease their fluid density, the next lower gas lift valve is operated to aerate the fluids above it and so forth down to the so-called "operating" valve of the well completion which is located in the region of the geological formation being produced. The operating valve operates to inject gas from the casing into the tubing to aerate fluids within the region of the production formation of the well and allow the free flow of fluids from the formation into the well.
The process of systematically aerating well fluids within the upper regions of the tubing down to the operating region of the tubing is called "lifting" the well, and removal of fluid in the annulus down to the operating region is referred to as "unloading" the well. When fluids accumulate within the tubing above the operating region it is necessary to periodically lift the well in order to have it operate in the most efficient manner Gas lift valves are the key components of a gas lift well completion.
As mentioned above, gas lift valves include either an internal nitrogen charged chamber aloting on a piston area or a compressed spring which holds the stem of the valve against the valve seat in order to keep it closed. In a tubing pressure operated gas lift valve, the pressure within the well tubing acts upon the piston area of the valve against the force of the spring. When the tubing pressure becomes sufficiently great (due to the accumulation of a column of fluid in the tubing) it will cause the valve stem to move upwardly opening the valve port and allowing gas to enter from the casing of the well into the tubing. In the conventional design of gas lift completions, it is generally assumed that a gas lift valve operates like a switch and goes from full closed to full open with no resistance to the flow through the valve. In fact, gas lift valves do not work this way. Instead, the valve stem moves from a closed position to an open position gradually and sometimes it takes a significant increase in the casing pressure in order to get the valve to move to the full open position. Thus, because of the gradual movement of the valve from a full closed position to a full open position, the amount of gas that is passed from the casing into the tubing through that valve varies depending upon the casing pressure, the tubing pressure, and the set pressure of the valve. The ability to accurately predict the rate of gas flow through gas lift valves has proven to be a very difficult task.
The use of gas lift valves in a gas lift well completion allows one to use relatively low injection pressures at the surface in order to overcome very high tubing pressures at great depths within the well, e.g., 9-10,000 feet. This is because the use of multiple gas lift valves spaced down the well allows it to unload fluids from the well in stages. The first valve unloads only the upper portion of the column of fluid in the annulus down to that first valve and the second valve unloads the fluids from the first valve to the second valve and so forth. This allows the use of a much lower casing pressure to unload the well because aeration of the fluids in the tubing above each valve reduces the tubing pressure at all the lower valves and allows those valves to be operated by a lower casing pressure.
In designing the placement of gas lift valves within a system and selecting the set pressures of individual gas injection valves in that system it is important to know how much gas will flow through the upper valve during the unloading process so that the tubing fluid gradient is lightened sufficiently to get down to the next lower valve. If the system designer does not know how much gas is actually flowed through the upper valve then either one of two inaccuracies is likely to occur. First, if the valve is flowing a great deal more gas than is anticipated, pressure of the injection gas in the annulus is pasted through the valve and is not available to continue unloading fluids from the annulus. Secondly, if there is not enough gas flowing through the upper gas lift valves the system will not be able to lighten the fluid gradient in the tubing above that valve sufficiently to be able to operate the next lower valve. Thus, it is important to accurately know the amount of gas which will flow through each gas lift valve in order to allow the unloading process to step down to the next lower valve. While inaccuracy in gas flow calculations is not a serious problem in the upper valves, because there is sufficient casing pressure to unload the fluid down to those levels, the problem becomes severe in the region of the lower valves. Especially in the region adjacent the operating valves at the lower part of the well it is very important to know how much gas has gone through each of the operating gas lift valves and what are the pressure conditions that exist while such flow is occurring.
The performance of a gas lift valve is basically a function of four parameters: (a) the set pressure of the valve; (b) the casing pressure operating opposite the valve; (c) the tubing pressure operating opposite the valve; and (d) the geometry of the valve itself, i.e., the position of the valve stem with respect to the seat at the various pressures.
Prior art techniques of predicting gas lift valve flow performance characteristics have been very imprecise. As mentioned above, one approach has been to assume that the gas lift valve operates either in a full open or a full closed position. While this assumption simplifies the prediction of the flows through the valve consideration, i.e., at full close the flow is 0 and at full open its flow is a function of the orifice size, it is in general a false assumption. Nevertheless, charts have been produced that predict gas flows through an orifice size based upon solely the upstream and downstream pressures. Such charts are relatively inaccurate and contribute to substantial errors in calculations connected with the design of gas lift completion systems.
In an effort to enhance the accuracy of the prediction of gas lift valve flow characteristics, another approach has been to try and experimentally analyze each valve at specific pressures and pressure differentials. While such actual test data is very helpful in the design of such systems, since the valve is measured under precisely the conditions at which it is to be operates, the data is very unreliable except at those very pressures. Further, the testing and preparation of charts for each valve at every conceivable operating pressure condition and pressure differential is exceedingly expensive and time consuming and impractical from a standard operating standpoint.
One other approach which has been tried in connection with predicting gas lift valve flow characteristics is that of running the valve at the pressures at which it is intended to be used and then determining a typical flow performance curve for the valve at that pressure. The flow performance curve is then broken down into two different portions. One portion is referred to as orifice flow and the other portion is referred to as throttling flow. For the orifice flow portion of the performance curve the flow characteristics are assumed to be simply that of orifice size. For the throttling flow portion of the curve, the gas flow characteristics are assumed to go from a high value to a closed condition over a range of 300 to 400 PSI and that the change from zero to maximum flow is linear as a function of pressure. Since the change in flow rate in the region of throttling flow is assumed to be linear, a slope is computed and used to calculate flow rates at other pressures other than at the specific measured values. This is a good approximation if the flow through the valve is linear, however, experience has shown that in the case of most gas lift valves the flow variation is far from liner.
When a pressure sensitive valve, such as a gas lift valve, is full open it has a certain fixed geometry which determines the flow rate through the valve in response to a pressure differential across the seat of the valve. However as the stem of the valve leaves its full open configuration and moves toward the seat, the geometry of the valve changes slightly which will, of course, affect the flow characteristics through the valve. As discussed above, accurate gas lift valve flow performance data is needed in order to accurately space the valves along the tubing of a gas lift completion. Virtually all spacing techniques assume that when gas is injected at a gas lift valve, the tubing flow gradient at that valve will be reduced according to a two phase vertical flowing gradient correlation. This reduction in tubing pressure will be reflected down the hole as a reduction in tubing pressure at each of the successively lower valves. The key to successive valve spacing in such a completion is accurately predicting the tubing pressure at each and every valve along the completion.
Since injected gas lightens the flowing tubing gradient, the differential pressure across the valve increases as the tubing pressure decreases. The rate of gas injection at each valve is the result of the individual valve's flow characteristics, the valve stem position and the differential pressure across that valve. As the tubing pressure decreases the valve stem will throttle to a closed position rather than going abruptly from a full open to a full close position.
Because of errors in the assumption that a gas lift valve moves abruptly from full open to closed position, the actual flow through a gas lift valve is considerably less than that which is calculated based upon this assumption. As a result, the assumed tubing pressure at the valve is much lower than the actual tubing pressure. Thus, an error in assumed tubing pressure causes unloading to become studied at an upper valve for lack of gas required to achieve the gas liquid ratio (GLR) necessary to uncover the next lower valve and creates a highly inefficient gas lift completion.
In except prior art gas lift valve performance testing to date, there have been performed a large number of tests on live gas lift valves in a fixture simulating a tubing side pocket mandrel at the pressures normally encountered in unloading operations. A plot of these flow rates versus tubing pressure is generally made and the data from these plots are then scrutinized to produce some type of empirical equation used to predict the maximum flow and the tubing pressure at which the maximum flow occurs through the valve. The resulting empirical equation does not usually include any specific valve parameters and is an equation only of a group of performance curves. This type of testing is time consuming expensive and applicable only to the specific valve tested and only at the specific pressure tested. Use of this type of empirical equation to try and predict flow rate performance at pressures other than those actually tested produces highly inaccurate and unreliable results in the design of gas lift systems. However, despite all its shortcomings, this type of testing and attempted empirical correlation is the technique most readily acceptable because it closely approximates what design engineers believe will happen downhole.
Further alternative to live valve testing is an attempt to theoretically analyze the stem position of the gas lift valve with respect to the port and then compute the pressure distribution throughout the valve. The stem position affects flow rates and pressure distributions and, conversely, the pressure distributions on the valve surface affects the stem position. Mathematically modeling this complex interaction of parameters is very difficult and highly prone to error. It has not yet been successfully accomplished.
The system of the present invention involves flow coefficient testing of valves in accordance with standard operating and testing procedures, well accepted in the industry.
The system of the present invention is designed to predict the flow performance characteristics of valves, or other flow control devices, that have a throttling character which comes about because the valves are pressure sensitive. Such pressure sensitivity arises both as a result of the absolute pressure of the valve as well as the differential pressure across the valve controlling the performance characteristics of the valve itself.
The system of the present invention enables the prediction of the flow performance of a valve with a minimum of actual testing of the valve. It also produces a continuous performance curve of the valve which enables a very precise prediction of the flow through the valve for various pressure conditions to which the valve may be subject.
The system of the present invention is adaptable to accurately measure a few limited parameters about a valve and then from that limited data accurately predict performance of the valve under numerous different and various operating conditions. This enables the designer of a gas lift system to very accurately predict the flow characteristics through the valve at virtually any design pressure condition within an operating environment.